The invention relates to a process for production of a sintered oxide ceramic of composition CexMyDzO2-a with dense structure without open porosity or with a predetermined porosity, where a first doping element M is used from at least one element of the group consisting of the rare earths but Mxe2x89xa0Ce, alkali and earth alkali metals, and a second doping element D of at least one metal but Dxe2x89xa0M.
It is known to produce ceramics based on ceroxide (Ce2xO) by means of various processes. Such processes are for example film casting or pressing with subsequent sintering at temperatures of 1300-1650xc2x0 C. Such high sintering temperatures are necessary to obtain a dense structure without open porosity and a high strength. However these high temperatures lead to a grain size in the structure of the order of several micrometers (xcexcm). As the grain size increases the mechanical properties of the ceramic deteriorate. The production and use of ceramics based on CeO2 is known from several points of literature, for example from Solid State Ionics, Vol. 36, 71-75 (1989).
WO, A 91/09430 describes a solid electrolyte composition based on ceroxide, a fuel cell and a process for generating electricity with this cell. The ceroxide ceramic contains a first doping element M selected from the group consisting of Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, Y, La and Ca, and a second doping element D from the group consisting of Pr, Sm, Eu, Tb, Fe, Co and Ni where Mxe2x89xa0D.
The first doping element M in WO, A 91/09430 is used to increase the ion conductivity, the second doping element D to reduce the electron conductivity of the composition under reducing conditions. The solid electrolyte is produced in the conventional way by means of pressing and sintering at temperatures in the range from 1300-1525xc2x0 C. No statements are made concerning the microstructure or grain size, but because of the parameters given, however, these must lie in the range of several micrometers.
Furthermore EP,A1 0722194 discloses the composition of a solid body electrolyte with defective fluorite structure for fuel cells based on ceroxide. A first doping element M is selected from the group consisting of Lu, Yb, Tm, Er, Y, Ho, Dy, Gd, Eu, Sm and Nd. Further doping elements A and B are alkali and earth alkali metals. The ceroxide composite materials described here have a lower electronic conductivity than conventional ceroxide electrolyte materials. The solid electrolyte is produced by pressing and sintering which is carried out at the usual high temperatures. A sintering process at 1500xc2x0 C. for four hours under atmospheric conditions is mentioned in order to obtain a sintered body with a cubic monophase fluorite structure. No statements are made on the microstructure or grain size, but because of the parameters given, however, a grain size of several micrometers can be assumed.
The inventors have faced the task of creating a process for production of a sintered oxide ceramic of the type described initially with which the sinter behaviour of CeO2-based ceramics can be influenced such that an extremely fine structure in the submicron range can be achieved with correspondingly good mechanical properties of the material. The electrical properties, i.e. the ion and electron conductivity, should remain unchanged despite the low grain size.
The task is solved according to the invention in that to the educt is added a second doping element D from at least one metal of the group consisting of Cu, Co, Ni, Fe and Mn, in the submicron particle size or as a salt solution, and sintered at a temperature of 750-1250xc2x0 C. into an oxide ceramic with extremely fine structure of a grain size of maximum around 0.5 xcexcm. Special and further developed embodiments of the process are the subject of dependent claims.
The first doping element M is added in order to obtain certain electrochemical properties for the proposed area of application, for example a specific oxygen ion conductivity. Common first doping elements M are known from the literature, in particular the rare earths such as La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, the alkali metals Ca, Sr and Ba and the earth alkali metals Sc, Y and Ga. These elements are used individually or in combinations of several elements.
For the process according to the invention commercial ceroxide can be used which already contains a corresponding mol fraction y of the first doping element M, which is advantageous in particular for routine applications. It is of essential importance that the powder is present in a particle size in the submicron range or is brought into this range. This can be achieved before or after addition of the second doping element D depending on the grain size of these components.
Evidently, for performance of the process according to the invention the individual components can also be used. Suitable starting materials, also known as educts, are in particular oxides of the components. These are ground dry and wet, then sintered. When inorganic salts are used as starting elements, wet chemical methods can be applied followed by coprecipitation, filtering and sintering.
The mol fraction x for the basic component Ce lies in the range of around 0.5 to around 1. The first doping element M is added in a mol fraction y of 0xe2x89xa6yxe2x89xa60.5. In the limit case the first doping element M can even be omitted. The mol fraction z for the second doping element D lies in the range of 0xe2x89xa6zxe2x89xa60.05. In particular the mol fraction z for the second doping element D, which is not optional like the first doping element M, lies in the range from 0.001xe2x89xa6zxe2x89xa60.02. Compared with the first doping element M, the doping range for the second doping element D is very narrow and close to 0.
In any case the particle size of the components used as educts for the oxide ceramic lies in the submicron range, suitably clearly below 1 xcexcm. This applies even though the expert knows that a certain proportion of coarser particles facilitates the handling of the powder. The preferred mean particle size of all components of the starting powder is below 0.1 xcexcm, in particular in the range from 0.01 to 0.05 xcexcm.
The sintering temperatures preferably lie in the range from 800 to 1200xc2x0 C., in particular 850-1100(copyright) C. Suitably a heating rate in the range of 0.5-20, preferably 1-10xc2x0 C./min is used. These sintering temperatures lie clearly below the previously conventional, even deemed necessary, temperature range above 1300xc2x0 C.
Depending on the required properties of the sintered ceramic, the sintering process is continued in a first variant until at least 98% of the theoretically achievable density is achieved. The sintering process according to the invention can however simply be continued within economic limits until at least 99% of the theoretically achievable density is achieved or even exceeded. The necessary process parameters are determined theoretically or empirically.
In a further variant of the process according to the invention porous sintered ceramics can also be produced. For a given porosity, for example 80% of the theoretically achievable density, the continuous temperature rise is interrupted at a temperature determined again theoretically or empirically and the sinter process terminated.
According to all variants the sinter temperature is preferably lowered again, for example by cooling, not immediately after reaching the end temperature but held at the predetermined optimum final temperature for a certain holding period. This holding time is preferably at least 0.25 h, in particular 1 to 2 h.
The advantages of the process according to the invention can be summarised briefly as follows:
Depending on the process parameters, a sintered oxide ceramic can be produced in dense structure without open porosity with more than around 98%, if necessary also more than around 99% of the theoretically achievable density. Premature interruption of the temperature rise during the sintering process can also lead to a porous structure with a specified lower density.
The sintered oxide ceramic has an extremely fine structure with a grain size of maximum around 0.5 xcexcm, preferably maximum around 0.1 xcexcm, whereby a material with better mechanical properties can be produced.
Due to the low sintering temperatures, the sintering process entails lower energy and investment costs.
The electrical properties with regard to the ion and electron conductivity remain good and unchanged.
The application possibilities of the ceramics produced according to the invention are correspondingly versatile, for example as electrolytes, electrodes, sensors and insulation materials.